The invention relates to a transmission, specifically an automatic, continuously variable transmission. Known transmissions of the kind discussed hereinafter have an rpm-converter means for converting an input rate of rotation (input rpm rate) to an output rpm rate, a transmission-control system for the rpm-converter means, as well as a supply circuit for the rpm-converter means into which a working fluid is fed by a fluid-conveyor device. The supply circuit carries the working fluid supplied by the pump by way of the transmission-control system to the rpm-converter means for setting different rpm-conversion ratios. A return circuit carries return fluid out of the rpm-converter means and/or the control system, e.g., to a reservoir tank for the working fluid. At least one branch conduit runs from the return circuit to provide at least one cooled and/or lubricated element of the transmission with working fluid. The transmission is further equipped with a flow-regulating device in the supply circuit, e.g., downstream of the fluid-conveyor device. The flow-regulating device has a run-off circuit to carry away the run-off portion of pressure that occurs as a result of the regulating function. The run-off circuit can lead to the reservoir tank or to the intake of the fluid-conveyor device to recharge the latter with working fluid. A transmission of this kind is shown, e.g., in DE 198 26 747 Al. A switching valve is arranged in the return circuit, whereby the working fluid flowing back from the rpm-converter means can either be directed to the intake of the fluid-conveyor device or supplied as a coolant to a clutch. The transmission described in DE 198 26 747 Al is preferably configured as a continuously variable transmission. Another continuously variable transmission is known, e.g. from DE 195 46 293 Al.
In automatic transmissions as described above, it is necessary to limit by means of the flow-regulating device the maximum rate at which working fluid flows to the transmission-control system and is made available to be directed to the rpm-converter means. The purpose of this regulation is to avoid excessive levels of backup pressure and losses in the conduits. In addition, the regulation allows the valves, particularly those of the transmission-control system, to be designed for a low flow rate, whereby the regulating performance is improved. As mentioned above, the run-off portion of the working fluid that occurs as a result of the regulating process can be used to charge the intake of the fluid-conveyor device, whereby the noise-generating properties of the fluid-conveyor device are improved. Thus, the flow-regulating device divides the total volume flow from the fluid-conveyor device into the regulated portion that is directed to the rpm-converter means and the run-off portion that is directed back to the reservoir tank or to the intake of the fluid-conveyor device. The working fluid returning from the high-pressure circuit (transmission-control system and rpm-converter means) is tapped off from the return circuit to be used with first priority for cooling and lubricating. Thus, the amount of fluid available as lubricant and coolant consists only of the remainder of the regulated portion after subtracting the control-system leakage and the dynamic flow-rate requirement of the rpm-converter means.
This creates a conflict between design objectives in transmissions of the known state of the art. On one hand, a sufficient amount of lubricant/coolant medium has to be available in order to avoid thermal instabilities in the thermal balance of the transmission even under worst-case conditions. This requires large quantities of fluid. On the other hand, there are also the objectives of keeping the pressure losses small and using compact dimensions in the transmission control system and its components, which requires relatively small quantities of fluid. Consequently, this causes a conflict because the control of the transmission requires a smaller amount of fluid than the lubricating and cooling functions.
It is therefore the object of the present invention to provide an automatic transmission that belongs to the kind of transmissions described above but does not have the disadvantage of the aforementioned design conflict.
The invention meets the foregoing objective in an automatic transmission with an rpm-converter means, a control system for the rpm-converter means, and a supply circuit leading to the rpm-converter means. A pump feeds working fluid to the supply circuit, and a return circuit carries spent working fluid back from the rpm-converter means and/or the control system. The transmission also has at least one cooled and/or lubricated element that is supplied by the return circuit. A flow-regulating device is arranged in the supply circuit, and a run-off circuit removes the run-off portion of working fluid from the flow-regulating device. In a transmission according to the invention, the return circuit coming from the rpm-converter means and/or the transmission-control system and the run-off circuit carrying the run-off fluid from the flow-regulating device merge at a circuit junction. After the confluence at the circuit junction, the united streams from the return and run-off circuits continue in a combined flow-back circuit. As a result of this arrangement, the regulated portion of the working fluid that is returned from the high-pressure domain and the run-off portion that is removed by the flow-regulating device are reunited. Thus, in addition to the remainder of the regulated portion after subtracting the control-system leakage and other usages of fluid, the run-off portion itself, which has been removed by the flow-regulating device, becomes available to be supplied to components that need lubricating and/or cooling. The total flow-rate of the flow-back circuit depends, as a first approximation, on the rpm rate of the engine to which the automatic transmission is connected. In power-transmitting elements, e.g., in the rpm-converter means, the required lubricant/coolant quantity, likewise, depends on the engine rpm rate, because the maximum amount of power to be transmitted increases in nearly linear proportion with the engine rpm rate. Thus, the available amount of working fluid is harmonically matched to the fluid requirement for lubricating and cooling, so that the heat that is being generated can be carried away by the lubricant/coolant medium even at high engine-rpm rates. In other words, although a constant rate of fluid flow can be made available for the transmission control system as well as the rpm-converter means over the entire rpm range of the engine, the lubricated and/or cooled elements can be supplied with a larger volume of working fluid depending on the engine rpm rate, as the fluid-conveyor device is driven at the rpm-rate of the engine and therefore recirculates increased amounts of fluid at higher engine-rpm rates, with the excess flow being directed to the run-off circuit at the flow-regulating device.
A preferred embodiment of the invention has a charging circuit that leads from the flow-back circuit to the intake portion of the pump. Thus, the flow-back circuit, which carries the working medium coming from the high-pressure area as well as the run-off from the flow-regulating device, also supplies the intake portion of the fluid-conveyor pump in addition to the cooled and/or lubricated element(s), so that the intake portion can be charged, whereby the noise-generating properties of the fluid-conveyor device are improved.
It is further preferred according to the invention to have at least one lubricant/coolant circuit branching off from the flow-back circuit and leading to the at least one cooled and/or lubricated element. Thus, both the at least one cooled and/or lubricated element and the intake portion of the fluid-conveyor device are supplied with working fluid from the flow-back circuit.
An especially preferred embodiment has a cooler and/or converter arranged in the return circuit upstream of the confluence of the return circuit and the run-off circuit. This prevents that too much working fluid is conducted through the converter or cooler. Particularly at times when the working fluid is at a cold temperature, the increased levels of back-up pressure could cause damage to the converter and/or cooler. However, by arranging the converter or cooler in the return circuit, which carries only the remainder of the regulated portion after subtracting the control-system leakage and the dynamic flow-rate requirements, the converter and/or cooler are protected against damage from excessive levels of fluid pressure.
It is, of course, possible to supply more than one cooled and/or lubricated element by way of the return circuit in an arrangement where each of the cooled and/or lubricated elements is supplied by coolant/lubricant circuit. Preferably in this arrangement, all cooled and/or lubricated elements are supplied in parallel out of the flow-back circuit. In a further developed version of the invention, the charging circuit is arranged to run parallel to the at least cooled and/or lubricated element. With the parallel arrangement of the cooled and/or lubricated elements and the charging circuit, it is possible to make the required amount of returning working fluid available as needed at each lubricated element.
A particularly preferred embodiment of the invention has a switching device arranged downstream of the circuit junction, i.e., in the flow-back circuit. The switching device serves to selectively direct at least a part of the working fluid in the flow-back circuit to flow either to a cooled and/or lubricated element or to the intake portion of the pump. This makes it possible to supply working fluid through the switching device to those lubricated elements that require working fluid only for short time intervals. If a lubricated element does not require fluid any longer, the fluid stream can be switched over to the charging circuit to resume charging the intake portion of the pump.
In an embodiment of the invention, the at least one cooled and/or lubricated element is a start-up clutch. As a particular feature of this embodiment, the witching device selectively supplies working medium either to the start-up clutch or to the intake portion of the fluid-conveyor device. This allows the start-up clutch to be supplied with lubricant/coolant fluid for short time intervals. The amount of lubricant/coolant fluid required for the start-up clutch is in direct proportion to the amount of torque to be transmitted and the slippage between the input and output of the start-up clutch. However the need for cooling fluid approaches zero as soon as the clutch is entirely engaged. Under these conditions, it can be particularly advantageous to supply the required quantity of coolant/lubricant by way of the switching device to the start-up clutch only during the time periods when it is needed and to use that fluid quantity for charging the fluid-conveyor device during the rest of the time.
A further developed embodiment of the invention has a hydraulic flow restrictor in each lubricant/coolant circuit and in the charging circuit. Through an appropriate design choice of the flow restrictor, e.g., a throttle, a narrow aperture or the like, each lubricated element can be supplied with the required amount of lubricant/coolant medium.
In embodiments of the invention that are equipped with the switching device, it is preferred if the flow restrictors in the clutch-cooling/lubricating circuit and in the charging circuit are dimensioned equally. Thus, the resultant total flow-circuit resistance is independent of the switch position of the switching device, which assures that the lubricated elements are supplied with the required amount of lubricant/coolant regardless of switch position.
In a preferred embodiment of the invention, the parallel-operating flow restrictors are dimensioned such that a minimally required system pressure of the working fluid will not be exceeded as a result of the combined hydraulic resistance. This means that the combined hydraulic resistance is designed so that the minimum system pressure can also be attained in the system. This can be acomplished, e.g., by arranging a pressure-limiting valve in the flow-back circuit, which releases the pressure in the flow-back circuit into an area of lower pressure. In particular, the pressure is released into the charging circuit, so that the run-off fluid from the pressure-limiting valve can be used to charge the fluid-conveyor device.
In a preferred embodiment, the switching device is controlled by a switching valve to selectively supply either the start-up clutch or the charging circuit.
In a particularly preferred embodiment, the switching device has a pressure-limiting function in addition to the switching function. This allows the quantity of lubricant/coolant fluid or charge fluid to be adjusted to demand. Preferably, the switching device is controlled by a proportional valve, so that a variable pressure limit can be set at the switching device. In particular, the switching device or, more specifically, the pressure-limiting function is controlled in function of the engine-rpm rate, the slippage of the start-up clutch and/or the speed of the vehicle that is equipped with the automatic transmission according to the invention. The setting of the pressure limit can be in a continuous range or in steps. The switching valve with the integrated pressure-limiting function is preferably designed as a switching valve with the pressure-limiting function as described in a great variety of embodiments in DE 199 22 232.0. The different embodiments of the switching valve with integrated pressure-limiting function are shown in FIGS. 8 through 14 of DE 199 22 232.0.
In a preferred embodiment of the invention, the automatic transmission is a continuously variable transmission, particularly of the kind where the rpm-converter means is a so-called cone-pulley belt-drive transmission.
If in the parked vehicle, the switching device is left in a position where the intake portion of the fluid-conveyor device is directly connected to a lubricated element, it is possible especially at cold temperatures that air will be sucked in through the lubricated element when the fluid-conveyor device is set in motion, i.e., when the vehicle is started and moved. To avoid this condition, the charging circuit is equipped with a threshold-pressure valve that opens the charging circuit only after a given amount of differential pressure has been attained between the flow-back circuit and the charging circuit.
The threshold-pressure valve is preferably configured as a seat valve to provide a tight closure of the intake portion of the fluid conveyor device against a lubricated element.
In a further developed embodiment, the threshold-pressure valve comprises a valve piston that is movable in its valve bore against the opposing force of a spring device in such a manner that in one piston position, the intake portion of the fluid conveyor device is closed off while in the other position, the charging circuit is set free.
Preferably, the piston of the threshold-pressure valve is configured as a stepped piston. Accordingly, the valve bore is designed as a stepped bore. This provides a simple way of arranging the seat of the threshold-pressure valve in the transitional area of the valve bore where the step of the valve piston meets the step of the valve bore, with the step of the piston carrying the seat face. The arrangement where the seat face, i.e., the sealing interface, is provided at the step of the piston, greatly facilitates the grinding of the sealing surface, e.g., with a stepped grinding wheel, resulting in a very good alignment of the piston face in relation to the piston axis.
If necessary, in view of the high level of surface pressure at the valve seat, at least the seat face can be configured as a wear-resistant insert component. With preference, both the valve piston and the housing that contains the valve bore are made from materials with matching coefficients of thermal expansion, so that even with variations in the working temperature, a sealing contact is maintained along the surface where the piston is guided in the valve bore, in order to avoid or at least minimize the leakage of working fluid.
In a preferred embodiment of the invention, the valve seat is recessed. In other words, the step in the valve bore is provided with a cutback along its internal circumference, whereby a ring-shaped step is formed with an open internal border towards the piston. This results in a good alignment of the edge of the valve seat in relation to the cylindrical valve bore.
Also, in preferred embodiments of the invention, the valve seat and the seat face have a conically sloped shape.
In a preferred embodiment, the insert component has the shape of a ring that can be slid onto the piston. With this arrangement, it is especially preferred for the valve piston to have a larger diameter at the side facing the valve seat than at the side that faces away from the valve seat, with the diameter at the side facing away from the valve seat being smaller than the internal ring diameter of the insert component. This allows the insert component to be easily slid over the end of the piston that faces away from the valve seat for the installation of the insert component on the step or, more specifically, on the seat face. Thus, the insert component can be made as a closed ring that is merely put in place on the piston. Under a special concept for the manufacture of the piston with the insert component, the insert component is first put in place on the step, i.e., at the seat face of the valve piston and, subsequently, the finishing operations are performed on the insert component together with the seat face of the piston.
The novel features that are considered as characteristic of the invention are set forth in particular in the appended claims. The improved apparatus itself, however, both as to its construction and its mode of operation, together with additional features and advantages thereof, will be best understood upon perusal of the following detailed description of certain presently preferred specific embodiments with reference to the accompanying drawing.